Demand for transportation fuels and other crude oil-based
products has been increasing. To meet growing demand for
cleaner fuels, refiners are using more severe
processing methods such as hydrocracking. Demand for refined
products has increased to the extent that refiners desire
larger hydrocracking reactors that can operate at higher
pressures with design conditions that are even more severe. New
feedstock diets for refineries
utilize more difficult to crack crudes; demand for
reactors that can withstand higher temperatures (over
450°C) and higher hydrogen partial pressures (12 MPa to 15
MPa) likewise is increasing. Under such severe processing
conditions, reactor vessels are constructed from low alloy
chromium (Cr)-molybdenum (Mo) steel of various grades.

Hydrocracking or cracking, in the presence of hydrogen or
dehydrogenating, is a catalytic process; heavy oils are
converted into lighter fractions. The upgrading is done by
several chemical reactions and involves the saturation of
aromatics, cracking (breaking the bonds of chains of Carbon NDR) and isomerization in the
presence of hydrogen. Hydrocracking is, therefore, one of the
two major conversion processes used by the modern refining industry. The other
important process is the fluid catalytic cracking (FCC).
However, this processing operation is mainly used to produce
gasoline.

Cracking operations play a more versatile role
in refining hydrocarbons. This process
can be adapted to produce middle distillates; thus, it is
widely adopted due to its ability to provide a wider range and
higher yield of quality products. Typical products from
hydrocracking include: liquefied petroleum gas (LPG), naphtha,
jet fuel (kerosine), diesel, ethylene, lubricating oils and
gasoline.

Process level.

The chemical reactions of the hydrocracking processes are
grouped into two broad classes. The first group includes
hydrotreating reaction, during which impuritiessuch as
nitrogen, sulfur, oxygen and metalsare removed from the
hydrocarbon mixture. The second group of reactions involves
hydrocracking, in which the carbon-carbon bonds are broken with the
help of hydrogen, using bifunctional catalysts. Typical
variability of hydrogen partial pressure in representative
applications is:

As shown here, the severe operating conditionshigh
temperature, high pressure and high partial pressure of
hydrogenincrease the activity of the catalysts. And only
under these harsh conditions can the best performance be
expected from catalyst materials; and therefore, the refining operations can be more
effective.

Designing for severe service.

Under these extreme conditions, reactors need to be
constructed from high-performance materials that are both
resistant to high pressure at high temperature and are
resilient to corrosive attack from the inside. In fact, the
majority of hydrocracking reactors in operation today, are
built from a low alloy Cr-Mo type 2.25Cr-1Mo steel. But the
trend in recent years is to build hydrocracking reactors with
materials with even better performance. A new generation of
steels such as low alloy Cr-Mo with enhanced vanadium (V)
2.25Cr-1Mo-0.25V [plate steel SA542 D4a and forgings SA336 F22V
(P-No.5C-ASME IX)].

Usage of better-performance materials has increased the
service life of high-pressure vessels and can exceed the
service life length as compared to those manufactured with more
conventional materials, even in cases where hydrogen partial
pressures are higher than those comparable to conventional
2.25Cr-1Mo reactors.

The industry has seen that 2.25Cr-1Mo-0.25V can provide
better mechanical properties at room temperature, hot and creep
as compared to conventional 2.25Cr-1Mo. In short, the
2.25Cr-1Mo-0.25V, when compared to conventional 2.25Cr-1Mo, is
considered to be:
 Stronger in tensile strength at elevated
temperatures
 Less vulnerable to temper embrittlement
 Less vulnerable to hydrogen embrittlement
 Less vulnerable to hydrogen attack
 More resistant to weld overlay disbonding.

The American Society of Mechanical Engineers (ASME) in the
Boiler and Pressure Vessel code has also recognized these
advantages. In the ASME VIII Division 2 Ed. 2007, these
additions are listed:

The significant revision of the allowable stress intensity
from the 2004 Edition to the 2007 Edition of the Code, shows,
at 454°C an increase of 18.2%, and, in the 2007 Edition,
shows an overall increase of the V-modified steel over
conventional material of 33.3%. This increase above
conventional material means that the V-modified steel will have
even greater application in the future.

Cost issues.

Thanks to all these benefits, reactors can be built lighter
and, therefore, cheaper. For the reactor manufacturer, this
translates into fewer and lighter movements in the factory,
easier transportation, lighter loads on the roads and lighter
lifting, which opens up crane availability and using a lighter
crane while loading on a ship or during erection, which means
less cost. The foundations where the reactor will sit can now
afford to be lighter and shallower. Each of these activities
provides cost benefits with a lighter weight reactor. The
industry cannot deny these benefits as they give considerable
economic advantage.

Manufacturers are becoming more confident in the construction of V-steel vessels and
are able to assist the engineering, procurement and
construction (EPC) companies in the evaluation of possible
alternatives, even hybrid solutions between plate and forgings
to assess the best results in terms of operational safety,
quality and cost.

Another aspect to consider is the inside surface protection.
Classically, hydrocracking reactors need to protect their inner
surface from direct contact from process fluids. This
protection is provided by an internal liner that is capable of
protecting the base metal from high-temperature corrosion. This
cladding is typically carried out by overlaying a weld metal
over the base metal. This process of weld overlay uses
austenitic stainless steel, usually the type SS 347,
niobium-stabilized to resist the phenomenon of precipitation of
carbides at the grain boundary, in particular, during
construction and, especially, during post-weld heat treatment
(PWHT).

However, the real purpose of the cladding is for process
service of the reactornamely preventing hydrogen
(H2) and other corrosive media attacks on the base
metal wall of the reactor. A major problem is that an
H2 attack can provoke:

 Decarburization of the surface as carbon
migrates to the surface of the material exposed to the process
fluids

 Carbon at the surface combines with
the free hydrogen to form methane (CH4) and causes
blistering on the undersurface (see Fig. 1).

Tough fabrication process.

Following so many positive characteristics, there must be
another side to this coin. And there is, in fact, the only weak
link in this design-materials-construction-in service chain is
limited to fabrication. All the potential risks are
borne by the manufacturer, so it is necessary to assign these
projects to reliable
manufacturersexperts with credentials. Some of the risks
in using 2.25Cr-1Mo-0.25V are:

o Cracking from not carrying out ISR for sufficient
time for nozzle welds

o Cracking resulting from weld flaw in nozzle welds

o Cracking resulting from cutting nozzle opening
through a bed support weld build up after DHT

 Field-weld repairs are much more difficult to
carry out, due to heating steps necessary in the welding
process.

Critical quality issues.

Material quality from the mill is critical;
consumable-material quality and management are also critical.
V-modified steel is difficult to work with and it needs to be
managed well. The manufacturer needs to properly plan the
construction of the reactor or vessel. From initial material
handling, through to cutting, rolling, beveling, welding, heat
treating and non-destructive testing (NDT) inspection, all need
to be tackled by skilled trained personnel.

Moisture problems. The real price to pay
for its advantages in mechanical properties is that V-modified
steel is extremely difficult to weld. To make the welding
easier, an increased overall material management system of the
welding process and welding consumables are necessary. In
particular, electrodes and flux are subject to intense drying,
between 350°C and 400°C, and maintained at temperatures
well above 130°C, to remove any sign of moisture. Moisture
is extremely harmful inside the welding process; moisture
contains hydrogenthe primary element for cracking. It is
imperative that even the welding material held in the welding
equipment during the feeding process of the weld should be kept
at elevated temperatures. The elevated temperatures help avoid
forming condensation and ensure that when weld consumable
material reaches the weld zone, it is dry and fully cleared of
moisture.

Managing the welding and controlling the heat treatments
helps to obtain the desired mechanical properties, especially
the required toughness. From historical evidence, typically the
heat-affected zone is the weakest area in most welded metals.
In V-modified steel, regarding toughness at low temperatures,
the critical zone is the area meltedthe weld deposit.
Today, despite all the technological efforts, the filler
material still faces some difficulties keeping up with the
requirements of industry.

The welding consumable materials are characterized by very
low storage of hydrogen, specifically designed for welding
steels with 2.25% Mo, 1% Cr, 0.25% V, resistant to creep and
hydrogen attack. The weld metal is resistant to embrittlement
caused by the high-temperature service, and is verified during
step cooling tests. The valuesX factor and
J factorare very low, on average below 15 and
100, respectively.

Another important factor in fabricating reactors in
2.25Cr-1Mo-0.25V is the PWHT. In fact, compared to the
conventional 2.25Cr-1Mo, V-modified steel requires a higher
temperature PWHT with longer holding times, typically 710°
+/5°C for 89 hours.

Critical temperature parameters.
Specialists in this field recognize that the weld metal on
these types of materials has a critical PWHT temperature of
705°C and a holding time at that temperature for at least 8
hours. These two parameters of temperature and holding time are
higher than the standard required by ASME where:

 Many specialists consider these temperatures and
holding times to be insufficient.

Therefore, to have good mechanical properties of materials
in welding, PWHT is carried out at higher temperatures and over
longer periods of time in special furnaces capable of treating
whole or sections of reactors from 800 tons to 900 tons.

In addition to these two parameters, the temperature profile
is critical. It is essential that the temperature is the same
all the way through the reactor body, and that during the
temperature rise and fall, the differences in metal temperature
is minimal.

Problems for reactors where temperatures of the PWHT are not
homogeneous can include:

 Potentially leave residual welding stresses and
generate new stresses due to the different temperatures in
various parts of the reactor

It is clear that the target must be to create a homogeneous
temperature profile over the whole reactor, where the
temperature gradient must be steady enough to ensure that
temperature differentials do not occur through the thickness of
the metal. Also, it can be demonstrated that the PWHT during construction plays a vital role in
determining the service life of the reactor, and that,
critically, any one activity can jeopardize the success of a project, but none more so than PWHT.
A well-executed PWHT can be proven to extend the service life
of the reactor.

Non-destructive testing.

Another very important aspect in the construction of the
reactors is nondestructive testing (NDT). For reactors in
2.25Cr-1Mo-0.25V, the acceptance criteria are necessarily
higher and more stringent than conventional steels. Even small
indications may give rise to problems later in fabrication,
where they can be a trigger for defects with greater
importance, such as cracks. Table 1 lists examples of typical
examination procedures used on certain weld types in the
manufacturing cycle.

Fitness for service.

A final consideration should be made to the minimum
pressurization temperature (MPT). Process equipment
fitness-for-service assessments using API RP 579 is a
sophisticated prediction tool to assess the metallurgical
condition of a section of process equipment. The analyses of
stresses and strains of pressure equipment can assist in
predicting whether operating equipment is fit for its intended
service. The studies predict how the material will behave
according to certain operating conditions and is used to
establish an MPT curve. This curve provides an accurate limit
for operating characteristics. In this manner, startup and
shutdown procedures can be set closer to these limits, making
the plant more flexible. If the MPT is under the curve, then we
are in optimal conditions. Other critical information necessary
to calculate the MPT include actual data from the material
used. There is a direct correlation between the X and J factors
and MPT. The lower the X and J factors, the lower the MPT. And
to achieve a low X and J factor, then cleaner materials with
fewer impurities are necessary. Fig. 2 shows a typical MPT
curve.

Fig.
2. Minimum pressure temperature
cure.

This field of research regarding the use of materials and
process standards for fabrication of heavy-wall vessels of
2¼ Cr-1Mo-¼ V alloy for service with hydrogen at
high pressures and temperatures is under continuous review.
American Petroleum Institutes publication of API 934-F is
under development exclusively for this topic.

Reactor fabrication.

In summary, there are only a small number of key factors
that greatly influence safer reactor fabrication. Intensive
training of all personnel involved in the fabrication and
inspection of the reactor is paramount. It is important that
each individual takes responsibility and care for himself, his
(her) fellow workers, as well as the entire team. The
importance of special care required in the management of
welding consumables is also illustrated here. We have
highlighted the importance of good reliable automated process
control during each of the welding phasesfrom
pre-heating, to welding, to post-weld heat treating. Control of
the complete manufacturing process should be guaranteed by
developing and following specific welding procedures, and by
fixing welding parameters in production with automatic
continuous recording and control methods. To minimize the risk
of premature brittle fracture, it is advisable to have an ISR
furnace in the shop. It is indispensable to be critical in NDT,
as small indications can propagate into larger failures.
Consequently, specific training and qualification are required
for all technicians and operators. Finally, and arguably most
importantly, it is important to have reliable execution of PWHT
procedures, with well controlled furnaces and skilled personnel
to guarantee precise temperature curves with a temperature
profile no greater than +/- 5°C. HP

Davide
Quintiliani is an international welding
technologist and international welding inspector, and II
Level of several NDE techniques. He joined Walter Tosto
in 1996 as a quality control Inspector; in 2004, he
became head of the quality control department with roles
of NDE and welding coordinator. From 2008 to present, he
is the head of the welding department, chief welding
coordinator and material selection specialist. Mr. Davide
has a degree from the University of Chieti G.
DAnnunzio, in health and safety at work and a
second degree in techniques of loss prevention at work
and the environment. He has authored
24 technical articles regarding PED, quality, NDE and
welding.

Giacomo
Fossataro is the technical and operation manager
at Walter Tosto with global responsibility for design,
manufacturing and quality control activities. He started
his professional career in Walter Tostos technical
department and has held many positions within Walter
Tosto including head of technical department and manager
of site activities. Mr. Fossataro holds a degree in
engineering (industrial technologies) from the
Politecnico di Milano.

Michael De Colellis is a project manager at Walter
Tosto SpA in Chieti, Italy. He has a BE degree in
manufacturing systems engineering from the University
of Hertfordshire, UK, and an MSc degree in advanced
manufacturing systems and technology, from the
University of Liverpool. Mr. De Colellis began in the
automotive and earthmoving equipment industry, working
from quality engineer to quality manager, at General
Motors and Case New Holand, before transferring into
the oil and gas pressure equipment construction
business.

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